1. Field of the Invention
The invention relates to a system for automatic correction of the color balance between the primary signals of a color-signal source.
2. Description of the Prior Art
From German Offenlegungsschrift No. 2,237,784, it is known to modify the chromaticity of a color-television picture by means of a color corrector. The known color corrector contains operating elements by means of which the chromaticity in certain luminosity areas of the color television picture is manually adjustable according to the subjective judgment of a viewer.
Furthermore, from German Offenlegungsschrift No. 2,141,685, it is know to carry out automatically color corrections of a color movie in a preprogrammed manner. It is necessary for such a preprogrammed color correction to have sufficient time for a test run of the color movie. During the test run, the chromatic aberrations determined subjectively by a viewer are stored scene by scene. During the broadcast, the individual scenes are corrected as a function of the stored chromatic aberration values. With certain color television broadcasts, such as news broadcasts with live insertions from other television stations, the required test run is not possible. Such live insertions are broadcast without having been previously viewed. Since, moreover, the scenes in newscasts change very often, the picture engineer does not have sufficient chance to remove recognized chromatic aberrations during the showing of the individual scenes.
To eliminate this disadvantage, U.S. Pat. No. 3,735,026 proposed an automatic color-correction system wherein the color balance of the red and blue chromaticity-value signals is automatically balanced with respect to the green color-value signal of a color-signal source. The balancing process takes place in this system when the amplitude of the luminance signal is smaller than 7% or larger than 95% of the maximum luminance-signal value, i.e. when it corresponds to the black or white values of the luminance signal. This balancing criterion rests on the assumption that the voltage difference of the color signals in black or white picture portions becomes zero. On the other hand, the existence of a color-signal difference in black or white picture portions shows that a picture stain (i.e., color tinge or suffusion of discoloration over the picture) exists, and therefore no color balance between the color signals exists. This system, however, exhibits undesired effects upon blue and skin-colored picture portions. A dark blue is seen as black and a bright yellow as white. Although no color stain in the color-television picture is present, the aforementioned balancing criterion alone would lead to further chromatic aberrations. For this reason it was further proposed to eliminate the balancing process in dark-blue and in bright-yellow hues. The hue-recognition circuits necessary for this purpose are, however, very expensive. Furthermore, the balancing area, already restricted to less than 7% and more than 95% of the luminance signal, is further restricted by this additional limitation. Because of the measurement of the brightest and the darkest picture points with respect to the voltage difference of their color signals, this procedure depends very much on the picture contents. Satisfactory color reproduction requires a correction of the gray values (gamma correction). This correction could hitherto not be carried out in red-green-blue (RGB) systems since, for this purpose, saturation information that is independent of luminosity is required.